A typical machine tool (e.g., a machining center) includes a spindle supported on a spindle head and various cutting tools attached to a tip of the spindle and is configured to cut a workpiece by rotating the spindle.
A choice of the various cutting tools depends on details of machining of the workpiece. A standardized holder (e.g., based on BT standard) is used for attaching variously shaped tools to a tip of the same spindle.
Each of the tools is fixed to the standardized holder and a chuck unit in accordance with the holder is provided to the spindle, so that the tools can be attached to the spindle irrespective of the shapes of the tools.
In the machine tool (e.g., the machining center), an automatic tool exchanger is used. The automatic tool exchanger is configured to reliably handle the tools, irrespective of the shapes of the tools, by gripping the standardized holder using a rotary arm and the like (see Document 1: JP-A-2009-39792).
However, in recent years, a machine tool configured to attach the tool to the spindle without using the holder has been used in response to demands for an increase in the speed rotation of the spindle and the accuracy of the machine tool.
For instance, in some machine tools, an air bearing is used for supporting the spindle to achieve a high speed rotation reaching several tens of thousands to several hundreds of thousands of rotation per minute (see Document 2: JP-A-09-257037).
Disadvantage of the above high-speed rotary machine tool is the use of the holder for attaching the tool. For instance, at an insufficient axis alignment accuracy between the tool and the holder fixed to the tool, vibration of the tool during the high speed rotation becomes hard, which is not suitable for the rotation support by an air static pressure bearing. Moreover, the high-speed rotary machine tool, which is often used for a highly accurate machining of fine shapes, is not suitable for being downsized since the machine tool requires the holder.
In such a background, the holder is not used for attaching the tool, but a straight shank of the tool is directly fixed to the spindle.
For instance, as the machine tool in which the tool is directly fixed to the spindle, a machine tool in which the tool is fixed by shrink fit has been developed (see Document 3: JP-A-2004-237408).
Moreover, as the machine tool in which the tool is directly fixed to the spindle, a machine tool in which the rotation support of the tool is separated from the rotation transfer from the spindle and a direct connection portion between the spindle and the tool mainly transfers only the rotation is also proposed (see Document 4: JP-A-2009-248224).
In the above machine tool, an air static pressure bearing connected to the spindle head is added to a tip of the spindle and supports the straight shank of the tool so that the tool is rotatable. A tapered end of the tool is brought into vacuum contact with the spindle, so that the tool is connected to the spindle.
In the above shrink fit as described in Document 3, the tool can be directly fixed to the spindle and a high axis accuracy can be secured. However, the shrink fit is required every time the tool is detached and attached, which requires heating and cooling time. For this reason, every time the tool is exchanged, the machining by the spindle is suspended for a long time to adversely lower an operation efficiency.
In the above arrangement as described in Document 4, the additional support structure is required and the support structure occupies a large space at the tip of the spindle. Accordingly, in addition to an increased restriction on machining, setting of the support structure is also restricted.
Further, a common problem of the arrangements to directly fix the tool to the spindle without using the holder is that a typical automatic tool exchanger configured to grip the standardized holder is unusable. Accordingly, for instance, it is necessary to move the spindle to a tool stocker and chuck the tool or to manually exchange the tool by an operator, so that an operation suspended time of the spindle is prolonged and unsuitable for increasing the operation efficiency.
In other words, without relying on the typical techniques described in Documents 3 and 4, a machine tool configured to directly fix the tool to the spindle and having a simple structure of the spindle has been desired.
Further, a machine tool configured to directly fix the tool to the spindle and capable of shortening the operation suspended time of the spindle at the exchange of the tool has been desired.
As a result of a dedicated study in response to such demands, the inventors developed a technique satisfying the above-described demands and filed a patent application as JP-A-2014-246055.
In the above patent application by the inventors, the following arrangement is employed in response to the demand for simplifying the structure.
A machine tool includes: a spindle rotatably supported on a spindle head; a chuck unit provided at a tip of the spindle; and a tool to be chucked to the chuck unit, in which the chuck unit includes: a cylindrical collet member into which the tool is insertible; a chuck hole that is open on an end surface of the spindle and in which the collet member is placeable; a squeezing mechanism that is formed between an outer circumference of the collet member and an inner circumference of the chuck hole and has a tapered surface extending from an opening of the chuck hole toward an innermost thereof and approaching a center axis of the spindle; and a pull-in mechanism configured to pull the collet member from the opening of the chuck hole toward the innermost thereof.
In the above machine tool, the tool can be clamped by being pulled in the axial direction of the spindle and squeezed over an entire circumference of the tool using the collet member, the chuck hole, the squeezing mechanism and the pull-in mechanism, so that the tool can be directly fixed to the spindle. Further, the structure of the spindle can be simplified without a mechanism projecting at the tip of the spindle.
In the above machine tool, the following arrangement is further employed in response to the demand for shortening the above suspended time.
The above machine tool further includes: a tool stocker that is provided out of a reach of the tip of the spindle and is capable of housing the tool; and a tool handler configured to attach the tool housed in the tool stocker to the spindle.
In such a machine tool, the tool handler can grip the tool housed in the tool stocker and attach the tool to the spindle. Accordingly, the operation of the spindle is not restrained except for time when the tool handler detaches and attaches the tool, so that duration when the operation of the spindle is suspended for tool exchange can be shortened.
Moreover, since the tool stocker can be disposed out of the reach of the tip of the spindle, the tool stocker neither interferes with the movement of the spindle nor narrows the movement range of the spindle.
Further, in the above machine tool, the tool handler includes: a tool holder configured to hold the tool; and a holder transfer mechanism configured to transfer the tool holder from the tool stocker to a reachable position of the spindle.
In the machine tool, the tool housed in the tool stocker can be held by the tool holder, the tool holder can be moved near the spindle by the holder transfer mechanism, and the tool can be attached to the spindle. On the other hand, the tool attached to the spindle can be collected by the tool holder and the tool holder can be transferred to the tool stocker by the holder transfer mechanism, whereby the tool can be returned to the tool stocker.
Accordingly, even when the tool stocker is disposed out of the reach of the tip of the spindle, detachment and attachment of the tool to the spindle can be reliably performed by the tool handler.
However, it is found that the use of the chuck unit, the tool handler and the tool stocker of the above machine tool by the inventors entails the following problems.
Specifically, in the tool handler, the tool is firmly held by the tool holder and the tool holder is transferred by the holder transfer mechanism to a tool exchange position (i.e., a reachable position of the spindle). For the tool exchange, the spindle is brought closer to the tool holder, the held tool is inserted into the collet member of the chuck unit, and the squeezing mechanism squeezes the tool to chuck.
At this time, even when the tool holder is placed in a predetermined tool exchange position, a slight axial misalignment is inevitable between a shaft center of the held tool and a shaft center of the chuck unit close to the tool.
Due to such axial misalignment, a deviated slide movement occurs between the tool and the collet member to decrease the holding accuracy of the tool by the chuck unit. Consequently, the tool chucked to the spindle is significantly vibrated in association with the rotation.
Moreover, at a large axial misalignment between the collet member and the tool, the tool cannot be smoothly inserted in the collet member, which may cause an operation failure at the tool exchange.
In addition to the above axial misalignment, from a viewpoint of the structure of the above chuck unit, when foreign substances (e.g., cut powders) enter the collet member, the chuck unit bites the foreign substances when clamping the tool, so that the holding accuracy of the tool cannot be secured and the vibration of the tool in association with the rotation occasionally becomes large.
The above problems of the chuck unit are common not only to the chuck unit including the collet member, the chuck hole, the squeezing mechanism and the pull-in mechanism as described in the above patent application by the inventor of the present application, but also to the chuck unit configured to attach the tool to the spindle in a manner to pull the tool therein in the axis direction of the spindle in order to directly fix the tool to the spindle.